Forschungsprojekte der Nanomechanics Gruppe

The impact of high current densities on the microstructure of nanocrystalline iron-based alloys and related effects during spark plasma sintering of these alloys

The spark plasma sintering method for preparing high quality oxide materials is applied here to the preparation of high performance nanocrystalline metals. Just as for the oxide materials, we aim to use electric fields and currents to enhance densification of metal powders while limiting grain growth. The goal of our study is to test how useful this method is for sintering metal powders and to obtain a fundamental understanding of the mechanisms controlling sintering with and without electric fields.

To start with, the advantage of electric field processing of metal powders is tested by characterizing the microstructure and mechanical properties of conventionally sintered and spark plasma sintered Fe(C) powders. The expectation is that the electric currents lead to electromigration-induced impurity and surface fluxes, thereby modifying the dynamics of morphology evolution and grain growth of the metal powders. The mechanisms underlying these effects are investigated in a set of control experiments of thin Fe(C) films where we can investigate the evolution of the microstructure, morphology, and impurities, with and without applied fields. The goal is to understand how impurity redistribution and resultant grain growth are affected by high electric current densitites.

Support of Priority Programme (SPP 1959) "Manipulation of matter controlled by electric and magnetic fields: Towards novel synthesis and processing routes of inorganic materials" by the DFG is gratefully acknowledged.

A wide range of studies show a dramatic effect of applied electric fields or currents on the sintering behavior of oxide ceramic powders. However, the mechanisms accounting for these strong effects remain elusive despite the wide application potential.

By using in-situ scanning and transmission electron microscopy, material changes during sintering of model oxide ceramics (such as ZnO) will be directly observed. The evolution of microstructure and morphology, including grain/void morphology, segregation, and precipitation, will be tracked, both with and without applied fields and currents. The results will be compared to models to determine the dominant driving forces and mechanisms for sintering, and how these are affected by fields and currents. In addition, in-situ high resolution and electron holography studies, EELS, and in-situ studies under oxidizing and reducing atmospheres will be used to gain insight into the atomic origins of sintering behavior, as well as of high conductivity states that occur in conjunction with flash events during field-assisted sintering.

Support of Priority Programme (SPP 1959) "Manipulation of matter controlled by electric and magnetic fields: Towards novel synthesis and processing routes of inorganic materials" by the DFG is gratefully acknowledged.

Grain refinement in ball-milled nanocrystalline iron-boron

In this project, iron-boron alloys were produced and the influence of the concentration of boron on the grain size was investigated. In theory, the relatively small boron atoms should segregate to the grain boundaries of iron. This reduces the surface energy of the grains and therefore favors the formation of grain boundaries and reduces grain growth. To investigate this, Fe and FeB powder were ball-milled to produce alloys with different boron concentrations. The grain size was determined by transmission electron microscopy and x-ray diffraction. It could be shown that in the investigated interval up to 17,3 at.% the grain size decreases with increasing boron concentration. The thermal stability of the powder was examined by differential scanning calorimetry, where a phase transition while heating and the formation of a Fe2B phase were observed. The results were connected to previous works on iron-oxygen and iron-carbon systems.

Understanding and controlling friction on the nanoscale

The dissipation of mechanical and kinetic energy at a sliding contact, better known as friction, has confounded mankind for centuries. Even though friction is often described by straight-forward empirical laws its fundamental cause is by no means simple. Despite the fact that understanding and controlling nanoscale friction is one of the major interests in modern tribology, it mostly remains elusive.

In this project, lateral force microscopy studies are carried out under UHV and ambient conditions using different material classes as model systems to investigate how the phonon, acoustic and electronic properties of a material affect its surface friction. One material class under investigation are mixed-valence manganite thin films that can be switched between a conducting and insulating state. These materials provide an ideal way to systematically vary the material electronic properties by external control parameters without changing the topology or chemistry of the sample surface.

Resistively switching a La0.55Ca0.45MnO3 thin films surface from an insulating state (IS) to a metallic state (LRS) under UHV conditions leads to a significant change in friction which is reversed on switching back to the insulating state (HRS) as shown by the average friction loops (left) and the average friction force against film resitance (right).

In-situ TEM study of LiMn2O4 lithiation

Lithium ion batteries are currently the most widely used portable and rechargeable electrochemical storage systems due to their high energy density. While there are many different Li-based cathode materials in use or being considered, spinel-type LiMn2O4 (LMO) provides a good model system to study electrochemical processes at a local scale. It has a strong coupling between structural and electronic/valence properties and is phase separating for higher contents of Li. This creates a solid/solid interface which allows both sides of an electrochemical reaction to be studied inside a high-resolution transition electron microscope (TEM). In combination with energy-loss spectroscopy (EELS) this makes a powerful tool to characterize sample structure, valence state and Li content at the atomic scale.

LixMn2O4 during in-situ lithiation in the TEM. The Li electrode with the native oxide/nitride is seen in the lower left corner of both images. (a) Bright field image taken immediately after contacting the lamella and applying a -5V bias. The dark regions result from bending. The region near the Li electrode has already been transformed and shows a sharp interface to the original material. (b) Bright field image taken after 180s of lithiation. The transformation of the lamella cut from the particle using focused ion beam machining is complete. The whole lamella now shows striped contrast with three different orientations across the whole lamella.

Previous studies on LiMn2O4 have been using XRD or XAS measurements or were limited to ex-situ TEM except for in-situ diffraction studies on LMO nanowires. Therefore, there is only limited information on the mechanisms of the phase transformation.
With our setup we are able to drive an electrochemical reaction in the LMO TEM specimen (Fig1) showing the formation of a lamellar structure. Using our FEI Titan to investigate the dynamics of the lithiation process, we have access to methods like high-resolution TEM, electron diffraction and EELS, that we can also combine with STEM to get localized information. With these methods we are able gain insights to the chemical and structural changes taking in the TEM lamella also giving us the possibility to look at local defects and interface structures. In this way we are able to observe a decline of the valence during the lithiation and find out that the lamellar structures fits to the expected tetragonal phase of higher lithium content while at the same time exhibiting a thus far not reported microstructure.

Quantitative heat capacity measurements of Au nanoparticles using a nanocalorimeter

As sample length scales reach down to nanometers, and the surface influence becomes large, thermal properties such as the heat capacity change. The involved heat in such samples is very small and can only be measured via convent. calorimetry by pressing milligrams of sample material together, whereby the surface influence of the sample is reduced. However, nanoscale samples can be directly measured by MEMS-based differential nanocalorimeters which have sufficient res. to measure with nJ sensitivity. Nonetheless, it is well known that heat losses of the MEMS sensors may change in the presence of a sample, leading to large changes in the nanocalorimetric signal and quantitative determination of heat capacity. In this study, FEA is used to investigate the interplay of heat losses (rad. and cond.), sample properties (therm. emissivity and conductivity), and temp. distr. on the sensor. On the basis of this analysis, the influence of exp. issues (misaligned sample, amb. temp.) can be evaluated. Furthermore, the sample geometry could be adapted to minimize the influence of the sample prop. on the measurement. With this knowledge, the heat capacity of Au nanoparticles (1 nm to 20 nm) was measured from 30 K to 300 K with a heating rate of 18,000 K/s with a resolution of better than 1 nJ/K.
Together with TEM investigations, this reveals an enhancement in the heat capacity at low temp. that decr. with incr. particle size.

Identifying structural changes in the medium range order of metallic glasses by Fluctuation Electron Microscopy (FEM)

The structural heterogeneity of a metallic glass (MG) is known to have tremendous impact on many important properties of the system, such as the mechanical performance and the formation of shear bands. While it is commonly understood how to qualitatively alter the structure by thermal and mechanical treatments, it is still extremely difficult to precisely quantify the subtle changes in the real-space order induced by structural alterations. For the characterization of medium range order (MRO) of an amorphous system, the method of Fluctuation Electron Microscopy (FEM) has been found useful in the past. In this method, very small volumes of the MG structure are probed by an electron beam in the Transmission Electron Microscope (TEM). Information about changes in the MRO is subsequently inferred from pixel variances of diffraction patterns taken at many different sample sites. We have used this technique to characterize the structure of a Pd77.5Cu6Si16.5 MG at different structural states induced by thermal annealing, cooling to cryogenic temperatures, and mechanical rejuvenation. The results will be compared with nanoindentation pop-in studies to understand the influence of structure on the onset of mechanical deformation.

Effect of hydrogen on the pop-in behavior in a metallic glass

Hydrogen is a detrimental element in structural materials which usually causes obvious reduction of fracture toughness. For example, hydrogen embrittlement is a well-known reason for failure of steels. In contrast, in metallic glasses, which are considered to be brittle materials,hydrogen (H) can play a beneficial roll by preventing shear band formation. In this contribution, the effect of H content on the popin behavior of a Zr based metallic glass (M) is studied. Samples are charged to different H content (H/M) with electrochemical hydrogen loading. Then, nanoindention with a spherical indenter is carried out on the surface of the samples. While the load of the first pop-in increases with increasing H content, the possibility of pop-ins occurring decreases. A transition from shear band dominated to homogeneous deformation is observed. This result is consistent with a study of the compression of submicron pillars in hydrogen gas environment. The suppression of shear band initiation is attributed to changes in the local environment induced by H doping, such as local strain.

Dynamic mechanical analysis of wood cell walls at the microscale and nanoscale

The hierarchical fiber composite structure of wood is optimized at all length scales for mechanical performance. Although mechanical properties of wood have been extensively studied at the macrostructural scale, knowledge of the microscopic mechanical behavior is essential to understand its outstanding functional mechanical properties. The wood cell wall is made up of three polymers; cellulose, hemicellulose, and lignin. Cellulose fibers with highly organized semi-crystalline structure and roughly 130 GPa Young’s modulus play the main role in cell wall stiffness. Lignin is an isotropic three-dimensional complex polymer with roughly 3 GPa Young’s modulus, and hemicellulose is a non-crystalline polysaccharide distributed inhomogeneously in the cell wall which binds cellulose to lignin by hydrogen and covalent bonds, respectively. To have a better insight into the mechanical behavior of wood cell walls, we are studying the mechanical response of Pine wood cell walls at the microscale with dynamic nanoindentation and at the nanoscale with dynamic Atomic Force Microscopy.

In-situ SEM / TEM fracture tests on (modified) pine sap wood

There is evidence that intrawall failure of wood occurs at the interface between the S1 and S2 cell wall layer. However, up to now, the fracture mechanisms of the cell wall remain unexplored. Therefore, we performed fracture tests of pine wood tracheid cell walls which are important because pine ? mostly made of tracheid cells ? is very common. Our experimental setup enables us to create a crack in the cell wall and to observe the crack propagation in-situ with an electron microscope. We have observed that crack propagation is not continuous, but intermittently starts and stops accompanied with a change in propagation direction. We attribute this intermittent behavior to the abrupt change of the microfibril angle at the interface between the S1 and S2 cell wall layer and propose that the resultant increase in toughness is a driving force for the natural adaptation of the layered structure. Additionally, we will investigate pine sap wood that has been heattreated or modified with the resin DMDHEU. The treated wood has higher durability but reduced bending and tensile strength relative to untreated wood. Here, the goal is to understand the reasons for the poorer mechanical properties on the basis of the crack behavior.

In-situ nanoindentation of thin films with STM-tips

The gerneral increase of the strength of metal with decreasing characteristic length is by now not completely understood. Especially the generalty over several orders of magnitude is difficlut to explain since the dislocation activity changes in nano scaled materials. One way to study dislocations is the investigation of the surface step that form when a dislocation reaches the surface of the sample. In this work we use an UHV scanning tunneling microscope (STM) to measure the surface topography of thin film on substrates before and after plastics deformation. The plastic deformation is done by performing (relativly uncontrolled) nanoindentations with the STM-Tip, using the same tip for imaging afterwards. By analysing step heights and directions one can gather information about the dislocation. In the future plastic deformation will also by done with an in-situ tensile loading system.